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User Manual
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2021-08-25
XDPP1100 technical reference manual
Digital power controller
Current sense (IS)
The selection over which switching cycle the current is averaged is done via register
isp_fsw_sync_sel
. The
user can decide whether the switching cycle is averaged over Loop 0 or Loop 1. The averaged current is further
processed downstream in the telemetry and fault blocks.
Note that the PMBus Page 0 commands for MFR_IOUT_APC and IOUT_CAL_OFFSET correspond to the ISEN
path, and the PMBus Page 1 commands correspond to the BISEN path.
3.3.2
Peak current limiting
PCL is implemented by comparing:
•
The reconstructed current waveform from the CE in IADC codes
•
PCL threshold,
ceX_current_limit
Register
ceX_current_limit
sets the cycle-by-cycle PCL threshold, which is defined in IADC codes. It can be
computed from amps according to Equation (3.12).
𝑐𝑒𝑋_𝑐𝑢𝑟𝑟𝑒𝑛𝑡_𝑙𝑖𝑚𝑖𝑡 =
𝐶𝑢𝑟𝑟𝑒𝑛𝑡 𝑖𝑛 𝐴𝑚𝑝𝑠
𝑀𝐹𝑅_𝐼𝑂𝑈𝑇_𝐴𝑃𝐶
,
(3.12)
The
ce0_current_limit
sets the limit for ISEN input, and correspondingly the
ce1_current_limit
sets the limit
for BISEN input. If the sensed current exceeds the current limit, the duty cycle is truncated by turning off the
PWM signal associated with the on-state (indicated by
ceX_on_mask0
and
ceX_on_mask1)
in order to prevent
the inductor current from increasing further. The PCL protection can be disabled by setting the threshold to
zero.
3.3.3
Negative current limiting
The negative current limiting (NCL) block is illustrated in
, and it compares:
•
Instantaneous phase current (prior to cycle averaging)
•
NCL threshold,
ispX_ncl_thresh
If the phase current drops below the NCL threshold, the FETs associated with the off-state (defined by
ceX_off_mask0
and
ceX_off_mask1
) are turned off.
In isolated topologies the FETs associated with the off-time are the synchronous FETs. In general, turning off
the SR FETs with a large negative inductor current is undesirable. Therefore, the recommendation is to disable
NCL in isolated topologies. This fault protection and its response can be disabled by setting the register
isp_ncl_thresh
to the maximum positive setting, 7Fh.
3.3.4
Short-circuit protection
Short-circuit protection (SCP) is implemented by comparing:
•
Instantaneous phase current (prior to cycle averaging)
•
SCP fault threshold,
isp_scp_thresh
For this SCP only a single sample above the threshold is needed to trip. Therefore, the threshold value should
be set the highest among the various overcurrent protection (OCP) thresholds (e.g., IOUT_OC_WARN_LIMIT,
IOUT_OC_FAULT_LIMIT, MFR_IOUT_OC_FAST_ FAULT_LIMIT).
In multiphase topologies, the SCP threshold is applied per phase, and the response to a short-circuit is to
immediately shut down the output of the loop on which the fault occurred. This is different from the PMBus-
based current faults, which operate on the total loop current. This protection can be disabled by setting the
threshold value in register
isp_scp_thresh
to zero.